This invention relates to thermal treatments of aluminum base alloys. More particularly, the invention relates to an improved aging process for an aluminum base alloy containing zinc, magnesium and copper as the alloying constituents.
Aluminum base alloys, particularly 7000 series alloys containing zinc, magnesium and copper, are conventionally solution heat treated at a temperature of from 750.degree.-1000.degree. F. The alloy is then quenched by exposure to cool air, hot water or cold water to retain a substantial portion of the dissolved components in a state of solid solution. The rate of quenching is influenced by several factors, including the possible inducement of residual stresses as well as the overall physical dimensions of the article to be quenched. Certain physical properties, particularly the tensile properties are dependent on the rate of quench. More particularly, the slower quench rates which may be necessary to avoid inducement of residual stresses or which may be necessary due to the physical bulk of the article, can significantly lower the tensile properties of the resultant article.
Conventionally, aluminum articles which have been heat treated and quenched are subsequently subjected to an aging process to enhance certain physical properties, including tensile properties. While aging, in its most simplified and traditional form, might simply involve allowing the material to remain at ambient temperature for a significant period of time prior to use, the more common and economically efficient practice today involves artificial aging. In an artificial aging practice, the heat treated and quenched material is maintained at an elevated temperature with respect to room temperature to accelerate the aging. For example, the aging temperature may range from 150.degree.-350.degree. F. The article is maintained at this temperature for a period of time of perhaps 4 to 24 hours and then allowed to cool to room temperature.
Some time ago, employees of the assignee of the present invention determined that significant improvements in certain physical properties, such as resistance to stress corrosion, cracking and tearing, could be improved if subsequent to the solution heat treatment and quenching the aging was carried out in two distinct steps at two different temperatures. Thus, in Sprowls et al, U.S. Pat. No. 3,198,676, assigned to the assignee of this invention, a two-step aging process is disclosed wherein the article is first aged at a temperature of from 175.degree. to 275.degree. F. for a period of from 3 to 30 hours (depending on the amount of zinc) followed by a subsequent aging step within the range of 315.degree. to 380.degree. F. for a period of from 2 to 100 hours. While the patentees disclosed a rather broad temperature range (175.degree. to 275.degree. F.) for the first step, in actual practice the patentees only illustrated aging carried out in the first step at a temperature range of from 225.degree. to 270.degree. F.
While the practice of the process disclosed in the aforementioned patent does result in enhanced properties, the overall tensile properties are still undesirably low for aluminum alloys which have been quenched at a slow rate.